The electronic compass has been integrated in various electronic products to improve performance. For example, the electronic compass can be used in the GPS to improve sensing ability. The heading direction in the GPS is determined by movement of object. However, when the object has low moving speed or even is in a static state, the GPS cannot precisely determine the orientation. The electronic compass can provide the information of the azimuth angle to indicate the direction.
The mechanism for sensing magnetic field has been proposed in various manners, such as typical Hall device or magneto-resistive devices. Magneto-resistive devices including anisotropic magneto-resistor (AMR), giant magneto-resistor (GMR) and tunneling magneto-resistor (TMR) have the benefits of larger sensitivity than Hall device, and the back-end process of the above described magneto-resistive devices can be easily integrated with the front-end process of CMOS.
A typical TMR for a magnetic field sensor 95 is shown in FIGS. 1A and 1B, including a bottom plate of conductive metal serving as a bottom electrode 102 formed on a substrate 90, a Magnetic Tunneling Junction (MTJ) device 110 formed on the bottom electrode 102, and a top plate of conductive material serving as a top electrode 106 formed on the MTJ device 110. From the structure pattern of a MTJ device, one can define a cross having intersection at the center of the MTJ device 110, the longer length is called as major axis 101 and the shorter length is called as minor axis 103, and also a line called easy-axis 180 is collinear with the major axis 101. The MTJ device 110 includes a pinned layer 112, a tunneling layer 115 and a free layer 116, in which the MTJ device 110 is sandwiched between the bottom electrode 102 and the top electrode 106. The pinned layer 112 is made of magnetic material formed on the bottom electrode 102 and has a first pinned magnetization 114 parallel to a pinned direction. The tunneling layer 115 of non-magnetic material is formed on the pinned layer 112. The free layer 116 of magnetic material is formed on the tunneling layer 115 and has a first free magnetization 118 which is initially parallel to the easy axis 180.
After the MTJ device is formed (i.e. completing magnetic thin film stacking and pattern etching), the pinned direction is set by applying a magnetic field perpendicular to the easy-axis 180 during an annealing process. After the annealing process, the pinned magnetization 114 will be parallel to the direction of the applied magnetic field, and the free magnetization 118 tends to be parallel to the easy-axis 180 due to the shape anisotropy of the MTJ device 110. Therefore, the magnetic field sensing direction of the TMR is perpendicular to the easy-axis 180 after the annealing process.
Through AMR or even GMR, it can achieve an integrated 2-axis magnetic field sensor, but the footprint sizes of them are quite large. Because of having very low resistivity, the device length has to be longer enough to a usable value for sensing magnetic field. FIGS. 2A and 2B are drawings, schematically illustrating a Wheatstone bridge circuit without and with shielding. As shown in FIG. 2A, the Wheatstone bridge circuit is a popular adopted method to transform the sensed magnetic field into an electronic signal. For the AMR magnetic sensor, each element R11, R21, R12, R22 of the bridge circuit is a series connection of several Barber pole biased AMRs and the shorting bar angles of adjacent elements are complementary, so that the bridge circuit is symmetric and full range operable. However, for the GMR or TMR magnetic field sensor, due to their symmetric magneto-resistance and magnetic field characteristics, two elements R21, R12 therefore must be shielded, as shown in FIG. 2B, and the bridge circuit only performs half range operation. For TMRs having high magneto-resistance ratio, the asymmetric half range operation of the bridge circuit results in losing linearity and accuracy to sense magnetic field by the bridge circuit.
U.S. patent application Ser. No. 13/097,083 entitled in “STRUCTURE OF TMR AND FABRICATION METHOD OF INTEGRATED 3-AXIS MAGNETIC HELD SENSOR AND SENSING CIRCUIT” discloses to magnetic field sensing devices having TMRs for sensing magnetic field. However, each X-axis sensor comprises two MTJ devices, each Y-axis sensor comprises two MTJ devices and each Z-axis sensor comprises four MTJ devices, based on FIGS. 6 and 9 in the US patent application. Further, two X-axis sensors, two Y-axis sensors or two Z-axis sensors are required to construct the magnetic field sensing device for magnetic field sensing, based on FIG. 12 in the US patent application. Therefore, the US patent application uses four MTJ devices for sensing magnetic field along the X-axis or Y-axis, and eight MTJ devices for sensing magnetic field along the Z-axis, thus increasing fabrication cost due to using too many MTJ devices, the mismatch between those MTJ devices may result in low yield.